1. Field of the Invention
The field of the present invention is vacuum boosters of a type in which a master cylinder for a brake or a clutch of an automobile is operated in a boosting manner by a vacuum pressure, and particularly, improvements of vacuum boosters comprising a rectangular pedestal which is formed bulged on an end wall of a booster shell and to which four connecting bolts are secured at four apexes of the rectangle surrounding an axis of the booster shell, the pedestal being attached to a vehicle body through the connecting bolts.
2. Description of the Prior Art
When the four connecting bolts for mounting the booster shell to a vehicle body are to be secured to a rear wall of the booster shell of the vacuum booster, the bolts are obliged to be disposed at four apexes of the rectangle owing to a structural restriction on the vehicle body. The pedestal formed on the end wall of the booster shell is thus formed into a corresponding rectangle surrounding the rectangle defined by the bolt (see Japanese Utility Model Application Laid-open No. 79058/82).
The present inventors have found that in a structure of the type described above, stress is concentrated around each corner of the rectangular pedestal when an axial tension load acts on the booster shell during a boosting operation. This is because the corners of the rectangular pedestal have a relatively large rigidity, while the central portions of its longer sides have a relatively small rigidity.